In various wiring installations, specifically in airframe or aircraft applications, the consequence of a fire or explosion resulting from an electrical arc propagation along the wire insulation is particularly serious. The insulation may be broken or damaged, exposing the wire in a number of ways, such as by the rubbing or chafing of the insulation along a sharp edge of the aircraft frame, or, in combat situations by unfriendly gunfire. When the insulation of a voltage-carrying wire is broken, subsequent contact of the exposed wire with another exposed wire or metal airframe member causes a short circuit which creates a large current discharge, generating an arc which melts the copper and decomposes the insulation into a conductive material such as carbon. This arc, in turn, generates sufficient energy to decompose or ablate the insulation of an adjacent wire. Clearly, if the adjoining insulation readily degrades to form conductive carbon paths and expose more wire after being subjected to the arc, the process of short circuiting can continue, increasing both the risk of electrical arcing and burning and/or explosion of flammable components in the vicinity.
There are several tests which measure resistance to arc propagation. Arc propagation resistance is tested under both dry and wet conditions. Dry arc testing is used to determine the ability of an insulation system to resist arc propagation resulting from a short circuit. Wet arc testing serves the function of determining the arc propagation resistance of the insulation system when an exposed conductor is subject to moisture which creates a conductive path. Several standardized tests have been developed to perform dry and wet arc testing, such as the SAE AS 4373 method 301 dry arc resistance and fault propagation and method 509 wet arc tracking, and the Boeing BMS 13--60 arc resistance. These test procedures are incorporated herein by reference.
Testing is typically performed on stranded copper wire having a metal coating which serves to protect the copper from oxidation, thereby improving solderability. If the insulated conductor is to have a 150.degree. C. rating, a coating of high purity tin, typically applied by electroplating, is used as the coating metal for the conductor. If the insulated conductor is to be rated for temperatures up to 200.degree. C., silver is used, and for ratings up to 260.degree. C., a nickel coating is used. Though the metal coating may be applied by dipping or other electroless method, the stranded copper wire is typically electroplated, and therefore will be described throughout as being plated with tin, silver or nickel.
One method of decreasing the risk of arc propagation is to increase the thickness of the insulation so that the arc duration and intensity is diminished. Further, because the distance between the adjacent wires is greater, the likelihood of damaging adjacent wires is decreased.
When the thickness of the insulation is increased, the insulation volume and weight typically also increase. Particularly in aircraft applications, but also for other uses of the insulated conductors where overall component weight and volume is critical, even small increases in volume or weight cannot be tolerated. Thus, the insulation must both protect against arc propagation and be of as low weight and dimension as possible.
One material having utility in improving the arc propagation resistance of the wire insulation is polytetrafluoroethylene (PTFE). PTFE is either applied to a wire as a tape which is wrapped on a bias with a certain degree of overlap, or as an extrusion, or as a coating over the wire. In either case, the PTFE is applied in the uncured, or unsintered, state. After the application, the PTFE is then sintered by application of heat.
During the sintering of the PTFE, the temperature of the environment during sintering must be greater than about 720.degree. F. (382.degree. C.). At these temperatures, the silver (200.degree. C. rating) and nickel (260.degree. C. rating) metal plating on the copper strand is not affected. However, tin (150.degree. C. rating) plating on the copper is affected in one of the following ways by high processing temperatures. Tin is the least expensive of the three metal coatings, but it melts at the relatively low temperature of about 232.degree. C. The tin plating will oxidize under the temperatures needed to sinter PTFE. This oxidation renders the surface resistant to soldering. Further, excess tin coating on the surface of the copper strand may melt and bond to adjacent strands. Finally, the processing temperature may be even sufficient to cause the tin to fully alloy with a portion of the copper strand, which also renders the wire resistant to soldering. The risk of temperature-related degradation is particularly acute where the insulation provides little heat protection, as where the diameter is small or the weight low, as required in aircraft applications.
Thus, one problem in insulated wire manufacture is the inability to use an unsintered PTFE layer over a tin-plated conductor, such as copper strand, where the temperature necessary for further processing of the PTFE layer heats the tin-plated conductor to temperatures sufficient to degrade the tin plating. There also remains the continuing problem of providing an arc propagation resistant insulated conductor having an insulation layer of minimized weight and diameter.
Therefore, one object of the invention is to provide an insulated conductor having a sintered PTFE outer layer where the conductor, such as copper strand, is plated with tin.
Another object is to provide an insulated conductor which is both arc propagation resistant and able to be used in applications requiring physical toughness together with minimum diameter and weight.
Yet another object of the invention is to provide a process for manufacturing arc propagation resistant tin-plated conductor having an arc propagation resistant insulation containing PTFE whereby the PTFE outer layer is sintered without degrading the tin coating on the conductor.